InSight Steers Toward Mars

NASA's InSight spacecraft is currently cruising to Mars. Yesterday, it performed its first course correction guiding it to the Red Planet.Credit: NASA/JPL-Caltech› Larger view

NASA's InSight lander has made its first course correction
toward Mars.

InSight, short for Interior Exploration using Seismic
Investigations, Geodesy and Heat Transport, is the first mission dedicated to
exploring the deep interior of Mars.

The lander is currently encapsulated in a protective
aeroshell, which launched on top of an Atlas V 401 rocket on May 5 from
Vandenberg Air Force Base in Central California. Yesterday, the spacecraft fired
its thrusters for the first time to change its flight path. This activity,
called a trajectory correction maneuver, will happen a maximum of six times to
guide the lander to Mars.

Every launch starts with a rocket. That's necessary to get a spacecraft
out past Earth's gravity -- but rockets don't complete the journey to other
planets. Before launch, every piece of hardware headed to Mars is cleaned,
limiting the number of Earth microbes that might travel on the spacecraft.
However, the rocket and its upper stage, called a Centaur, don't get the same
special treatment.

As a result, Mars
launches involve aiming the rocket just off-target so that it flies off into
space. Separately, the spacecraft performs a series of trajectory correction
maneuvers guiding it to the Red Planet. This makes sure that only the clean
spacecraft lands on the planet, while the upper stage does not come close.

Precise calculations are required for InSight to arrive at
exactly the right spot in Mars' atmosphere at exactly the right time, resulting
in a landing on Nov. 26. Every step of the way, a team of navigators estimates
the position and velocity of the spacecraft. Then they design maneuvers to
deliver it to an entry point at Mars. That navigation team is based at NASA's
Jet Propulsion Laboratory in Pasadena, California, which leads the InSight
mission.

"This first maneuver is the largest we'll
conduct," said Fernando Abilleira of JPL, InSight's Deputy Mission Design
and Navigation Manager. "The thrusters will fire for about 40 seconds to
impart a velocity change of 3.8 meters per second [8.5 mph] to the spacecraft. That
will put us in the right ballpark as we aim for Mars."

Especially at the beginning of that cruise, navigators rely
on NASA's Deep Space Network (DSN) to track the spacecraft. The DSN is a system
of antennas located at three sites around the Earth. As the planet rotates,
each of these sites comes into range of NASA's spacecraft, pinging them with
radio signals to track their positions. The antennas also send and receive data
this way.

The DSN can give very accurate measurements about spacecraft
position and velocity. But predicting where InSight will be after it fires its
thrusters requires lots of modeling, Abilleira said. As the cruise to Mars
progresses, navigators have more information about the forces acting on a
spacecraft. That lets them further refine their models. Combined with DSN
tracking measurements, these models allow them to precisely drive the
spacecraft to the desired entry point.

"Navigation is all about statistics, probability and
uncertainty," Abilleira said. "As we gather more information on the
forces acting on the spacecraft, we can better predict how it's moving and how
future maneuvers will affect its path."

Yesterday's 40-second burn relies on four of eight
thrusters on the spacecraft. A separate group of four is autonomously fired on
a daily basis to keep the spacecraft's solar panels trained on the Sun and its
antennas pointed at Earth. While necessary to maintain orientation, these small,
daily firings also introduce errors that navigators have to account for and
counterbalance.

"Everyone has been working hard since launch to
assess what these small forces have done to the trajectory," said Allen
Halsell of JPL, InSight's navigation team chief. "People have worked lots
of hours to look at that. For engineers, it's a very interesting problem, and
fun to try to figure out."

When the spacecraft is just a few hours from Mars, the
planet's gravitational pull, or gravity well, will begin to reel the spacecraft
in. At that point, InSight's team will prepare for the next milestone after
cruise: entering Mars' atmosphere, descending to the surface and sticking
InSight's landing.

JPL, a division of
Caltech in Pasadena, California, manages InSight for NASA's Science Mission
Directorate in Washington. InSight is
part of NASA's Discovery Program, managed by the agency's Marshall Space Flight
Center in Huntsville, Alabama. The InSight spacecraft, including cruise stage
and lander, was built and tested by Lockheed Martin Space in Denver.